South Korea blocked anti-Pyongyang activists from access to a border region Monday amid heightened military tensions in the wake of North Korea's threat to strike the area if the activists send propaganda leaflets into the communist nation.

About 80 defectors had planned to send 200,000 leaflets by balloons from Imjingak near the Demilitarized Zone at around 11:00 a.m. despite drizzling rain. They were not able to hold the event as planned, however, as the military and police banned their access to the site starting early in the morning, in addition to tourists, residents and journalists.

"We have placed an entry ban on civilians and vehicles taking into consideration safety concerns over North Korean warnings, clashes between anti- and pro- civic groups and complaints from residents," said an official at the Gyeonggi Regional Police Agency.

The activists protested the entry ban.

"North Korea's threat is aimed at instigating division between people in the South," said Park Sang-hak of the Fighters for Free North Korea, who organized the event.

The South Korean military has stepped up combat readiness by deploying artillery and tank brigades and combat air patrols by F-15K and KF-16, according military officials.

"If (the North) launches attacks, (the South Korean) military will strongly and thoroughly retaliate against the origin of the attacks and their supporting forces under the right of self-defense," said Kim Min-seok, the defense ministry spokesman, in a briefing. "We are closely watching the North Korean military's movements."

On Friday, North Korea said its army will launch a "merciless military strike" if any move to drop leaflets is detected. South Korea's defense minister reacted swiftly, saying his military is prepared to "completely destroy" the origin of a North Korean attack if it occurs.

The North Korean threat came a day after South Korean President Lee Myung-Bak made a surprise visit to the front-line island of Yeonpyeong in the tensely guarded Yellow Sea, which was shelled nearly two years ago by the long-time rival.

Civic groups in the South have sent anti-Pyongyang leaflets in the past, but the communist state's unusually strong threat of an attack is the first since North Korean leader Kim Jong-un succeeded his father Kim Jong-il last December.

Pyongyang has condemned the leaflet drop as psychological warfare and an attempt to topple its communist regime, warning it could ignite a war on the Korean Peninsula, though it did not actually launch an attack.

The two Koreas still remain technically at war, as the 1950-53 Korean War ended in a truce, not a peace treaty. (Yonhap)